Assume part of your PT with subtotals
looks like this:

Sum of Value Type
Contract Date BTU Weight
A11 1/2/2006 187 299
1/3/2006 96 152
A11 Total 283 451
A13 1/2/2006 92 147
1/4/2006 96 158
A13 Total 188 305
A14 1/1/2006 93 140
1/2/2006 95 144
1/3/2006 92 144
A14 Total 280 428
B11 1/2/2006 96 155
1/3/2006 92 149
1/4/2006 188 299
B11 Total 376 603

When you do
PT toolbar Formulas Calculated Item
you will see no reference to the subtotals,
so you cannot generate your desired formula.
However, if you generate another PT, but this time
leave only Contract in ROW, then you will get
only the subtotals:

Sum of Value Type
Contract BTU Weight Bt/Wt
A11 283 451 0.63
A13 188 305 0.62
A14 280 428 0.65
B11 376 603 0.62

Then add the field Bt/Wt by clicking on Weight, then
Formulas Calculated Item (not Field) Formula:
=BTU/Weight
Name: Bt/Wt or any name you like.