It's definitely worth measuring the load to see how accurate it is, especially up at 29MHz when using the diode or scope method.
With the scope or diode method the following will apply:
A 10% error in the load (eg 55 ohm) will mean about a 10% error in the power calculation.
A 3% voltage error with the old analogue scope would mean about a 6% error in the power calculation.
A -30dBc harmonic in the source could mean about +/-6% uncertainty in the power calculation.
A -40dBc harmonic in the source could mean about +/-2% uncertainty in the power calculation.
Using a x10 scope probe is a bit dodgy because it will have to be compensated correctly and the response known to be flat to 30MHz and beyond. If it peaks up sharply at VHF then it could make the error conributions worse from harmonics up at VHF. Also, I wouldn't want to use a x10 scope probe at 100W at 29MHz because a typical low cost x10 scope probe will be starting to look like a dissipative load by 29MHz.
Don't use a scope directly with a tee piece above about 10-15Vrms if operating across the HF band. I would advise to never connect an HF transmitter directly to a scope even if it is only a few watts. It's not worth the risk, especially if the load accidentally disconnects and the voltage at the scope input goes up. The scope will look like a dissipative load by 29MHz and parts inside could get stressed. It gets much worse by 50MHz.
If you use the diode detector method then use a decent Schottky diode like the HP2800 or 1N5711. Don't use one of the cheap/classic germanium glass diodes, the detector performance up in the linear region will usually be inferior.
However, even if you ignore all the advice above you might get away without damaging your scope and it probably doesn't matter (in the real world) if the error contributions above meant you actually adjusted the meter to make it slightly worse than it was originally
Maybe it would be a good idea to measure the trimmer resistors in the meter with a DMM before adjusting them. So at least it could be put back where it was if things don't work out.