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Instruction for installing a RCexl ignition without use of a degree wheel
First install a piston stop into the spark plug hole

The piston stop is a simple bolt that matches your spark plug size and thread pattern

CM6 spark plug--10mmx1mm thread pattern bolt

BMMR6A-BMR6A--14mm x 1.25mm
Thread pattern bolt.
Install piston stop into cylinder spark plug hole
Now rotate the crank counter clockwise until piston hits the piston stop

Put a mark inline with each other on the prop hub and the crankcase
Now rotate the crank clockwise until piston hits the piston stop

Put a mark on the crankcase inline with mark on the prop hub.
Put a mark halfway between the two marks on the crankcase.

That will be the top dead center mark for timing the engine
A big thanks to Ralph Cunningham (Call name Antique on RC Universe.) For furnishing the information below
Measure the diameter of the prop hub and multiply by .244 for 28 degrees before top dead center
1.572 x .244= .384" Set you calipers for that setting
Put one point of the calipers on the top dead center mark on the crankcase and then mark the other end on the crank case, That will be your 28 degree before top dead center mark.
Rotate prop hub and line up mark on prop hub with the last mark you made on the crankcase and the engine will be now setting at 28 degrees before top dead center, When you rotate the prop hub counter clockwise and magnet passes the sensor the ignition should fire when the prop hub mark and the 28 degree mark line match up.   
Here is a alternate way to figure your timing mark if you want to set it other than 28 degrees before top dead center.

Multiply the diameter by 3.14 and then divide that by 360, Multiply that by what degree you want to set the engine at, In this case it will be x 30

In the case above to be set at 30 degrees it would be .410 in place of .384