I took some pics when I got home from work today with a carrier and barrel (with the upper receiver removed) to give you a better idea of what you are attempting to measure.
This is the distance you are trying to measure in relation to the hammer dropping. In general it should be 0.1 or less for proper timing. If this gap is too big when the hammer falls, the hammer can "catch up" to the bolt carrier prior to it closing this distance resulting in a less than 100% clean strike on the firing pin.
Here is how it is effectively being measured via inserting a drill bit.
Here is the issue I am talking about when trying to insert a drill bit through the ejection port or the magwell as there can be material in the way keeping you from getting a good measurement.
In both cases the drill bit is going to be shimmed against the upper receiver vs. be sandwiched flat between the carrier and barrel extension. How "off" the gap measurement is varies from upper to upper. However it generally gives you a false high reading. i.e. you put a 0.09 drill bit in the ejection port and the hammer wont fall because the drill bit is resting on the ejection port making the gap measurement 0.011
I made a set of short rods in varying diameters (0.125", 0.11", 0.9", 0.07", 0.06).
I then use a rubber band held into the carrier via the cotter pin to hold the shim to the carrier.
You can then put the carrier into the upper, reattach the upper to the gun, and cycle the charging handle to confirm if the hammer will fall with the varying diameter shims.
edited to add when reattaching the upper to test you need to do the "RLL upper receiver install" method to the lower where you remove both the front and rear lower receiver takedown pins and guide the upper receiver rear lug into the lower first, then lower the front of the upper onto the lower and reinsert both upper and lower take down pins. The reason being is that the bolt carrier will be slight slightly sticking out the back of the upper because of the shim so you cant just pivot the upper closed as you normally would.
If the hammer falls on the 0.125 or the 0.11 shim you know the sear is timed to early and the hammer is dropping before the carrier is closed enough. The other smaller rods can be used to confirm the actual timing of the gun. Ideal would be to be to close on the 0.11 and not have the hammer fall but close on the 0.09 rod/shim and have the hammer fall.
I have personally never really had an issue with a hammer falling too late as long as bolt bounce is properly controlled. However with a DIAS you can shim it to be right in the perfect range as needed.
In regards to light primer strikes there can be a bunch of other reasons for light primer strikes other than sear timing. Headspace, bolt bounce, hammer spring power, etc. My first thought on an M16 with a suppressor that is getting less than 100% primer strikes is bolt bounce. However, it certainly cant hurt to confirm you DIAS timing as well. If you have a dremel tool you can cut down some drill bits from home depot and then all you need is a rubber band (and a bent paperclip to hook it and pull it out the front of the carrier)
If you have any other questions please let me know.
Hope this helps