The problem with alkaline batteries in these heads is that they have a completely different power delivery curve compared to the original mercury cells, which falls further and further from the calibration spec as they age in service. These old meters relied upon the battery not only for voltage but for a specific delivery curve, they didn't have voltage regulators in their circuits. So what happens is that the meter reading can be right on the money some times, and other times well off the mark. Nothing is less useful than an inconsistent meter.
I'm also not certain that the MR-9 battery converter works well in the FTn heads. There are two batteries in series in these heads, I seem to recall hearing that there were issues.
The best course of action is to have the meter circuit re-calibrated to use currently available batteries. Since I re-acquired a Nikon F Photomic FTn last year, I hunted around and found Sover Wong, an F2 specialist. We've only talked about it so far, but he said he could work on F meter heads and provide the same services he does for F2 heads. You can find him at
http://www.soverf2repair.webs.com/ ..
(I haven't had mine done as I have been using the plain prism finder and a hand held...
🙂
G