Lets use a 3-9x50 scope for example:  = 50mm lens

1:  measure your lens  = 50mm

2:  half it   = 25mm

3:  check measurment of mount in product info (in mm from center of mount to base)

4:  mount measurment is 40mm

5:  add mount 40mm to half lens 25mm  = 65mm

6:  means maximum 65mm lens

from centre of mount to base of mount it is 40mm
your scope is 50mm  so from centre to edge = 25mm
should give you aprox 15mm gap from scope lens to barrel .

Please allow for items like Dust, lens covers, fixed sight hood on end of barrel