Hello,

TREND is IMHO not the right way to go. If, for example, you would have
A values of -1,000,000 for the points 6.5 and 6.8 then TREND through
all points 6.5, ..., 6.8 would result in -493,126 for the three points
6.6, 6.67 and 6.7.

If I understood you correctly you were looking for an interpolation.
My formula is giving you exactly that.

An interpolation UDF you can find he
http://www.sulprobil.com/html/interpolate.html

Regards,
Bernd