Using ST fuel trim to determine AFR???

That just tells you what is commanded at WOT, not what you are actually getting. Actually the calculation is correct, except it's not short term trim times lambda value, it's stoich point of the fuel times lambda - but in your calculation it is correct because you used the stoich point of gasoline: 14.64. That's really what tuning is partially about, getting your actual A/F to be the same as commanded A/F. You can sometimes extrapolate what the short term trims are doing at idle and part throttle to WOT IF the MAF you are usuing has a fairly linear output, but certainly not always. Nine times out of ten, corrections must be made at WOT even if short term trims are in line.
Don