Seasoning
% off combined meat weight
Pork + Belly500 g1.10 lb
Salt1.8%9.00 g0.32 oz
MSG0.4%2.00 g0.07 oz
I+G0.03%0.150 g0.01 oz
Black pepper0.3%1.50 g0.05 oz
Sage0.2%1.00 g0.04 oz
Red pepper flakes0.3%1.50 g0.05 oz
Cayenne0.1%0.500 g0.02 oz

Start with the combined weight of your deboned pork butt and added pork belly. The full spicy breakfast sausage formula scales from there.
A loose 20–30% belly target works well. No pork-to-belly ratio is assumed in the math.
finished sausage mix
From 500 g combined meat · 8.93% additions% off combined meat weight
Add after the dry seasoning
Work the mixture until it turns tacky, then stop before the fat warms and smears.