confusion with heat loss analysis for concrete slab on grade

I'm in the early planning stage for a detached steel woodworking shop with a 4" slab on grade floor and heated by a hydronic radiant system, using embedded 1/2" PEX. Building will be 24W x 30DP x 8.5' H. The slab will have 2" Extruded foam beneath and along the side going down 2' against the footer (24" frost line ). I'm trying to calculate my heat loss using a couple different sources I found on line. One is taken from the Design and Install manual for Hydronic Radiant Heating Systems, by Vanguard Pipe and the other is at a site called Build It Solar. Both seem to be pretty thorough. The Vanguard worksheet has an additional feature, whereby you enter the U factor for every component and then multiply it by the Design Temp Diff. which gives a Heat Loss Multiplier. You then multiply each components' square footage by that multiplier to get the Btu loss. My confusion is with the input entry for the slab itself. The Vanguard asks for U factor for the slab edge, which I found elsewhere to be .53, applied to the perimeter. The BIS app also asks for the R value of the slab per foot of perimeter, which I converted from the U factor to get 1.88. But both also have an input for a floor R value or U factor. Can't figure out if I need to enter something there as well and what it should be. Any help is greatly appreciated as I haven't found an explanation for this anywhere else. Thanks in advance.