You experience is a common one. Most shops don't have the proper equipment and just guess at it by listening to the sound change as the timing changes. No need to give a shop money to just guess at your timing. You could do that for free at home.
The other problem is the meters needed to time these are hard to find. A good number of member have used a different but accurate way to time these without the traditional diesel timing memter.
They use a digital timing light and a ferret adaptor. The ferret clamps to the #1 injection line and picks up the pulse of fuel during injection and translates that into a signal that can be used on a timing light. You use the timing light to tell where your timing is at by watching the timing mark on the harmonic balancer and adjusting the light pulse up or down. When the mark is at zero on the balancer that is where your timing is set. I know this sounds very confusing. Sorry. It's much easier to understand if you see that setup in action. Maybe someone can explain the ferret adaptor better for you.
Heath