Transformation coefficients. Am I doing it right?

Hello all!

I have, quite recently, been affected with the "photometry bug". Up till today, I have been using iTelescopes and Sierra stars for that purpose. 
Last week, I received my new ccd-cam (QSI6893) and BVR-filters. (Oh, happy day!) My telescope is a Skywatcher Explorer 190MN.

To get things right from the beginning, I'm trying to determine the transformation coefficients for my system. Due to the fact that I'm a newbie, can someone take a look att my procedure and results? It's a lot to learn, mostly in a non native laguage, and my mathematic knowledge is somewhat out of date. Better to discover my mistakes before reporting any observations.

I have taken photos of ngc7790 and Landolt SA114. The images have been calibrated (bias-dark-flat), platesolved and uploaded to vphot. In vphot I have loaded the compstars and created a sequence with only compstars to read the instrumental magnitudes. The images for ngc7790 was taken in an airmass of ~1, FWHM ~3. The images for Landolt SA114 in an airmass of ~2, FWHM ~5-6. The Landolts is a little bit low in my sky, but I'm living on North 60. 

I'm attaching a spreadsheet with my measurements and calculations. The values for Tbv and Tvr seems to look quite similar for the two different fields. The values for Tb, Tv and Tr seems to be a little bit more scattered. Maybe that will be averaged out after analysing more standard fields? Also the value for Tr seems a little bit high, have I missed something?

So again, I would be very happy to have this checked.

Clear skies,

Tomas Wikander
Borlange Sweden
WTHB