If you're mixing POTA, I presume you're planning to develop either Tech
Pan or a microfilm stock of some kind.  I don't have my books with me
(the movers still have them), but the figure you give for POTA seems
about right.  Don't forget that phenidone is the only developing agent
in POTA, and is used in conditions of controlled exhaustion in order to
reduce contrast.  In most other developers where it appears, phenidone
is used in conjunction with hydroquinone and/or metol  Phenidone is
regenerated by the hydroquinone, so much less phenidone is needed in
formulae that combine the two, and when phenidone is added to a a metol
formule, hydroquinone is almost always present.

That said, there are a number of other formulae that are more
predictable and consistent than POTA, even if you're not measuring your
1.5 g of phenidone with a spoon.  I'd recommend trying HC-110 Dilution
G, with reduced agitation; I've found it to work well with Copex Rapid
even at EI 100 (IIRC, 11 minutes at 68 F, constant agitation first
minute, then agitation every third minute).  For small quantities (less
than the 8 ounces or so needed for a single 35 mm film) it's easier to
get repeatable mixes if you mix from stock solution; the amount of syrup
is so small that size of drop that hangs on your syringe might produce a
visible change in your negatives.

Another alternate is Caffenol LC, 2 tsp Folgers coffee crystals and 2
tsp washing soda (I use Arm & Hammer Super Washing Soda) in 8 ounces
water; Copex Rapid needs about 40 minutes with agitation every minute to
get EI 100, but gives very nice gradation; extreme enlargement will show
more grain than with HC-110, but not much more.  Dissolve the carbonate
first, and stir until completely clear before adding the coffee, then
stir until all grittiness is gone.  The stuff smells pretty bad, but
does a nice job on microfilm and should work on Tech Pan as well.