250-500cc TDC timing tool for my WXC 360, good idea?

Hoping to get some opinions on the accuracy/reliability of using a simple TDC timing tool such as this one: https://www.ebay.com/itm/Bultaco-Os...e=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649

I installed a powerdynamo ignition on my '92 360, the timing procedure is relatively simple: mount stator plate, set piston to correct position (1mm/.040" BTDC in this case), align magneto timing mark with stator timing mark, hold flywheel and torque. I used a very rudimentary method to set the timing consisting of inserting a wooden dowel down plug hole, mark TDC with razor, measure up 1mm from TDC and make another mark.

It got the job done for all intents and purposes, bike starts easily and runs pretty well. Although I'm hesitant to really hammer on it because I'm skeptical of how accurate my timing method was. Inspecting the plug reveals an asymmetrical ignition pattern (uneven light brown only partially coloring plug), an indication of detonation/pinging perhaps? As mentioned the bike seems to run well to me, perhaps my ear is not trained well enough to identify detonation?

I surmise using a dial indicator to set the piston position then somehow holding the piston in place would be the ideal method to set timing. Problem is that there is no room for a dial indicator with the motor in place, at least not any that I have attempted to use. My question is: What tool(s) do you folks recommend I use to reliably set my timing with the motor in frame? Also: Anyone in close proximity to Asheville, NC with experience in this matter who's willing to barter their assistance/knowledge for beer/food/gas money would be more than welcome...ha!

that would work...but im not sure how you would zero it? i guess you could just do the math..

You make a valid point regarding the lack of capability to "zero" the gauge. I have seen timing gauges that can be zeroed then the correct BTDC measurement can be dialed in and locked in order to hold piston in place. Perhaps I'll hold out for such a tool. Upon further consideration the tool I posted a link to is essentially a glorified wooden dowel.

it is tight to fit a gauge in there
as I recall I used one with a small dial

Yes, insufficient clearance for a dial indicator, at least the one I attempted to use. It was a motorcycle specific, genuine Yamaha dial, had some age to it, may have been better suited for air-cooled motors. I'll also keep an eye out for a small dial.

this one fit in the chassis of my 93, I used it to check and set piston squish

Good to know there are dials out there that will fit with the motor in frame. Guess I'll have to take some measurements and cross reference my findings to various dial dimensions. Thanks for the pic.

it is snug but it fits