Correction
I realise that both "TAS/10 + 7" (mine) and "TAS/10 +5" (gdpf's) are incorrect as formulae.
The true formula is:
TAS/10 (KTS) plus 50% of the value left.
Thus, "+7" is good for 140KTS and "+5" for 100KTS.
Apparently, "+5" can be used formulaically when using MPH.
Rgds
CB