I ELF'ed boulder garden yesterday and agree with your 150-200cfs assessment. NF does seem to pick up quite a bit of water by the time it hits Waterton. Then ran Deckers and it was padded out nicely probably at least 450-500 based on past runs.
BTW, the gauge that the AW page uses for "Deckers" which I always used to use is total BS - its actually the outflow from Cheeseman ... Don't ever look at that one, use USGS Trumbull for the Deckers run: USGS Current Conditions for USGS 06701900 SOUTH PLATTE RIVER BLW BRUSH CRK NEAR TRUMBULL, CO
. Case in point, today AW Deckers says 263 whereas Trumbull is 439.
So, lets look at the math:
Trumbull: 439 cfs
North Fork at bailey: 63 cfs
Waterton: 910 cfs
408 cfs unaccounted for in Waterton
Lets say 1/2 of this is north fork, 1/2 is main fork. Sounds like North may be more as discussed above, but based on what i saw yesterday its not more than about 200 cfs up at Foxton town.
Do the math (408/2 + 63= 269 cfs) and we have a decent estimate for NF flows at the confluence, but there are some inflows between Foxton and Confluence, lets say 75-100 cfs which you could subtract.
So, my unscientific BS formula for estimating Foxton flows is:
(Waterton-Trumbull-Bailey)/2 + Bailey
A visual gauge at the T would be great.