Thermowell U length Calculation?

Have you searched ?

Most people just use a measuring device of some sort, Guvnor, and the thermowell manufacturer's catalogue information. You don't want that, though, do you?

it is more complicated to calculate, than measure:

Measure constant temperature at different insertions. Make diagram. From diagram You can see, at what insertion error is acceptable for Your application.

However, from calculations You can lear much also. I recommend to do estimations.