I don't recall your plumbing setup, so my comments will be theoretical. Your boiler is rated at 200,000 btuh. I will assume that is maximum at high burn, delta-T=20F, return water at 160F, supply at 180F, gpm=20. I also will assume that average output over a burn load is about 150,000 btuh, which at the same parameters = 15gpm.
An appropriate setup (look at your manual for specifics) would be return water protection no less than 140F, and up to 160F would be good and might be better. Mine is about 145F minimum no matter how much below this is the system return water temperature. Also, boiler circulator should be set to not turn "on" until boiler temperature is 160F or somewhat higher, and should turn off if boiler supply drops below 160F. I have a sensor fixed to the boiler supply pipe just as it exits the boiler, and my "on" point is 165F at this location, which means at initial "on" actual internal boiler temperature is somewhat higher.
I think a reasonable goal for delta-T between boiler supply and boiler return after return water protection should be 20F, at least until system return water before protection reaches about 165-170F, assuming for your boiler 20 gpm flow rate and assuming a high medium to high burn rate. This means that at 20 gpm your circulator can move 100% of boiler output all the way up to the point when the boiler will shut down (assume 190F). For example, supply 160F and 140F after protection, supply 170F and 150F after protection, etc., until supply 190F and 170F after protection (which also means that return protection valve should be wide open because system return before protection is above 160F).
As system return before protection rises (in the above example) above 170F and a high burn rate continues, a "problem" begins to occur, because at boiler shutdown at 190F, delta-T is now less than 20F, and if gpm = 20, flow no longer can handle full boiler output. Assuming continuing high medium to high burn, your boiler will start idling cycles. At delta-T=15, you only are moving 150,000 btuh with output as high as 200,000 buth, thus boiler heats to idle point and idling begins until boiler cools down. There are at least 2 solutions to this "problem," the easy one being loading your boiler so that as system return (which also would be return from storage) approaches 170F, the boiler wood load is burning down to medium burn and below, which means that boiler output is less than 200,000 btuh and falling, and at 20 gpm full output is still being moved from the boiler. Experienced or careful loading (as I achieve with weighed wood burns) can result in the entire storage tank being brought up to 190F (no system load) as the wood load burns down and then out.
A second solution is to increase gpm as delta-T is less than 20F. A variable speed circulator might do this, or installing in parallel a second circulator can also do this. A second circulator could be set to "on" at 185F and "off" at 175F, for example. Actual increase in gpm will depend on pump head at the increased flow rate. I used the second circulator solution in the past and it worked well, up to its limit on additional flow rate.
You don't necessarily need 20 gpm flow rate, although with your boiler that is what I would plan for with plumbing and circulator selection. If your flow rate is less than 20 gpm, idling will become more likely because boiler output can be greater than the system will draw and boiler temperature will rise, and also delta-T will be greater than 20F. This is not an issue at lower return water temperatures: If your flow rate is 13 gpm and delta-T=30F, you still are moving 200,000 btuh. Example: return water after protection 150F, boiler supply 180F, gpm=13, system will move 200,000 btuh. But you can see this becomes an issue as boiler supply approaches your idle point of 190F, and idling may start to occur at system return temperature of 160F.
Similarly, if your flow is greater than 20 gpm, that too is not necessarily an issue. If your flow rate is 25 gpm, idling will become less likely because boiler output will be less than the system will draw up to a higher temperature, and also delta-T will be less than 20F. This too will not be an issue at lower return water temperatures: If your flow rate is 25 gpm and delta-T=16F, you still are moving 200,000 btuh. Example: return water after protection 150F, boiler supply 166F, gpm=25, system will move 200,000 btuh. And the greater flow rate will allow full boiler output at 190F up to 174F system return water before idling will begin.
I would think that with your system, a base flow rate of much less than 20 gpm is unsatisfactory due to the extent of idling that is likely to occur. Also, a base flow rate of much more than 20 gpm is inefficient, except as suggested with a variable speed or parallel circulator design. In all cases you will want to load and fire your boiler to minimize idling based on system demand and actual flow rate. Hope this helps.