Nandu, why not just leave the new instrument and the commercial RF
ammeter connected in series, and then adjust the power level (by detuning)
of the power source (transmitter) and make a calibration table by comparing
known current to that which you are reading on dial of new instrument?

Some of us "old school" would make a new instrument dial by
disassembling the instrument, and reversing the dial plate (flipping it
over).
or creating a new paper dial plate with a computer, and glueing it over the
old one.

Good luck,
Lynn, W7LTQ since December 1948