Can anyone help me find out what the outer diameter size if of the standard Getrag crownwheel?

I have fitted a Qauife dog box with 'rally' ratios. The diameter of this crownwheel is larger - and obstructs the vehicle speed sensor that counts the teeth and hence drives the speedo. My plan is to mill down the sensor and see if it still works?

I know the size of the crownwheel I have, but not of the original - so cant calculate how much I need to take off other than trial and error. And I'm getting a bit tired of the 'error' part of that route....

cheers, Colin