Hi,
I always set the mesh using the good old fashioned paper trick between the pinion and min gear.
What I do is cut a strip of paper and feed in between the pinion and main gear, then I push the motor assembly towards the main gear taking care to push the motor and lower bearing block squarely towards the gear. With pressure on the motor assembly I tighten up the securing bolts and then remove the paper.
Hope this helps,
Ross