Dear Craig,
Thanks for being so reasonable, and not taking offence.
With that metering technique, yes, your EI strikes me as entirely believable, because you are only measuring the illumination in either case, not the light actually reflected from the darkest part of the subject. You are also going on what you like best, which as I said, normally implies maybe 1/3 stop extra for most people. The latter knocks the EI down to 100, and another 2/3 stop for not metering the shadows seems likely.
On the other hand, I normally develop FP4 in DD-X (true IS0 an easy 200) and rate it at 125 when spot metering. If I place the darkest shadows with detail 3 stops down (i.e. set the meter at 1000 on my Gossen Spotmaster 2) I get what I want in the way of shadow detail. Going 3 stops down arguably underexposes by about 1/3 stop, so I'm overexposing by about 1/3 stop. If I use the Pentax spot meter instead of the Gossen, and use the I.R.E. 1 index, I can afford to rate the film at 160. In D76 I'd expect an easy ISO 125 (HP4 generally runs a tiny bit fast) so I'd use 650 on the Gossen and 100 on the Pentax.
As I say, I had no intention of being unduly aggressive or pedantic, but although ISO film speeds are not perfect, they are the best we have. To refer to anything else as a 'true' speed is less than fair, because too much depends on individial technique and preference. I can rate FP4 at anything from 1/3 stop to a full stop faster than you, and, I would cheerfully bet, get exactly the same shadow detail, because of variations in metering technique and developer choice; but ISO speeds are replicable by anyone, unlike my speeds and yours.
Cheers,
Roger