Conversion factors.

Got it.
A small chart to compare tubes.
[img]https://dl.dropboxusercontent.com/u/3572198/Chart%20-%20GM%20all%20tubes.png[/img]

To save the calculations for you.. :P 
0.0039

Hello guys, thought this would be handy to post newly calculated conversion rates. :)
M4011:
Test info taken from here: http://tskagawa.com/?page_id=2061

Known:
Conversion rate for SBM-20 --> 0.0057

Calculations:
uranium glass bead 238U :
SBM-20 --> 290cpm * 0.0057 = 1.653µSv/h
M4011 --> 1.653µSv/h / 245cpm = 0.0067469387755102

uranium ore 238U :
SBM-20 --> 471cpm * 0.0057 = 2.6847µSv/h
M4011 --> 2.6847µSv/h / 436cpm =  0.0061575688073394

smoke detector pellet 241Am :
SBM-20 --> 896cpm  * 0.0057 = 5.1072µSv/h
M4011 --> 5.1072µSv/h / 790cpm = 0.0064648101265823

lantern mantle 232Th full size:
SBM-20 --> 5253cpm * 0.0057 = 29.9421µSv/h
M4011 --> 29.9421µSv/h / 4386cpm = 0.0068267441860465

Averaging:
0.0067469387755102 + 0.0061575688073394 + 0.0064648101265823 + 0.0068267441860465 = 0.0261960618954784

0.0261960618954784 / 4 = 0.0065490154738696

0.0065490154738696 <-- rounding to 4 digits behind the dot --> 0.0066 <-- Theoretical conversion rate for the M4011 after applying maths. :)

Hope this helped out. :)