Whatever AC amperage you have coming from grid-tie inverter, thermal-magnetic breaker should be 125% of that, to prevent nuisance trips (due to hot weather and wires heating breaker.)
The Sunny Boy parameters may be in terms of watts not amps. If you divide assumed watts by 240V, amps will likely be that number or less, because utility usually drives line voltage a bit high and it rises higher with inverter backfeeding.
In other words, don't worry about inverter documentation's call for wire and breaker, just dial down output wattage until existing wire and breaker is OK. (Of course, wire ampacity must be sufficient for breaker used.)
I don't know how durable MPP is, but Sunny Boy and Sunny Island are rock solid.
If you're off-grid, then Sunny Island supposedly can manage Sunny Boys up to 2x the wattage of Sunny Island.
I've messed around a bit with transformers between SI and SB (or TriPower), can off advice if you try it.
MPPT will take the place of your math, although with math you can estimate how much clipping and extended hours.
My plan is to put 15kW of PV panels on one 7.7kW SB for my sister's house.
Midnight has SCC for 150, 200, 250, and now 600V.
But if you're off-grid, you don't have to get SB -41 for its grid features. Look for earlier transformerless or the transformer type 5000 ~ 8000US. Just set them for offgrid (no RS-485 interface needed for use like in backup systems, just use to configure.) That series can have PV array negative or positive grounded, preventing "PID" for panels which are more susceptible. I've bought older SB for $0.10/W.