On the cat 10 I bought last fall, I had the magneto and the carb rebuilt.  Got them back this week.  

I have the magneto set to just start to open the points when the timing mark for the mag is lined up on the front pulley, like the book says.

 I rigged up a fuel line, a filter, and fed it some gas with a small funnel into the filter.  Got it started for about 20 second to make sure it has oil pressure. I need to adjust the carburetor, and rig up a small temporary gas tank.  No antifreeze in it yet.

After I get a temporary gas tank in place, will a timing light help to fine tune the timing?  It has an American Bosch mag with the coupling that can be adjusted 1 spline tooth at a time. Or is getting the marks lined up close enough and move on to the next item?

On a car I would just twist the distributor a bit and listen to the motor.  I never got into setting point dwell, etc.

you can use a timing light to set the ignition timing even with a magneto you just need a battery to power the light. the inductive pick up still clips on to plug wire one.

you need the mag gear aligned to the marks on the mag shaft then align that to the marks on the engine and fly wheel when cyl one is at tdc on compression all distributor style ignition engines are like this.

to find if cyl one is on tdc on compression for 4 stroke engines remove the rocker cover and watch for the change over of the valves on cyl 4 ie the exhuast will be closing the other opening. all 4 strokes are similar the cylinders work in pairs with 4,6,8.10.12 cyl engines find which is the mate to no1 and you can find the compression stroke, generally its the third cyl in the firing order.

This should help.
these pictures are from the PT10 book and the PV15 book. 

Thank for the reply. I have a reprint book and I timed like the book says. But I'm more or less asking, after setting the timing by the book, then using a timing light, can a guy get the timing more accurate than just trying to see when the points "just start to open". Or is the method in the book close enough, and I shouldn't fiddle with it as it will not help how it runs?

a timing light would be more accurate. but these arent a car or race engine so setting it by eye is good enough. also timing lights didnt exist when these old girls rolled off the line