Pressure Vessel Baseplate Calculation

I am using Dennis Moss's "Pressure Vessel Design Manual" for baseplate calculation. On Ed 3 Page 197-198, it have two formula for baseplate thickness for type 3 and 4 design (between gussets and between chair) and I assume we need to take the bigger value. The moment for "between gusset" formula use the maximum bolt load (P=FsxRa). My question is, why we need to use the bolt load for baseplate calculation? Isn't the bolt load acting on the top plate/top ring? And how to determine the allowable tension stress of anchor bolt (Fs), is it some percentage of yield stress, or use the allowable design stress from ASME Section II Part D?

Thanks.