With one and a half inches center to center. The shift levers I have built have three positions, in half-in intervals, starting at 1.5 inches. This gives you a bit of adjustability.
Most important is to ensure that the actuator runs full depth on both contraction and extension. Second most important thing is to ensure that the actuator rod is absolutely 90 degrees to the shift arm when at rest. If you have good upshift, but less-than-great downshifts, look to ensure that the arm is moving to its full extension. If not, you will have troubles.
As well, ensure that you isolate the actuator cylinder as best you can from heat sources (exhaust) The actuator is filled with a viscous material that will melt and clog the cylinder if overheated (common problem). If so, send it to me and I will repair it for you. I service these units regularly.
Once I repair them, they will not seize again in use.
These systems work GREAT when set up. Most all the problems are due to the physical setup on the engine. Get that correct, and you will have no problems; if you don't get it right, well, you will be frustrated.
Any ?? you can PM me and I will do my best to help. I have done a "few" of these installations. I also have spares in stock.