I've been using this formula for K.
K = R2 * CM
R2 = R1[1 + TCR(T2 - 75) From: NEC Tbl.8 Note 2.
TCR = Temperature Coefficient of Resistance, cu = 0.00323, al = 0.00330
T2 = new temperature, usually 60 or 90
CM = circular mils
Except for percentage adjustments in Tbl.8, Note 3, this formula does not differentiate between stranded and solid AWG. Tbl.8, Note 1, seems to assume normal-stranded wire for the chosen TCR's, unless differences between strands and solids was considered negligible.
To incorporate Bob's formula correctly for solid AWG, it looks like I could multiply my result above by 0.98 ?