So let me get this straight what you're asking is:

Say you attack someone heavily and go from 10 planets to 100 planets. Then a tick passes. If, in the next tick, they retake and you go from 100 to 50 planets, will you get put in pmode?

The answer to that is that I got no idea. I've seen people enter pmode without even losing half their planets before.

example

tic(T) you have 120 planets
tic(T-48) you had 80 planets
tic(T-47) you had 82 planets

if you lose 40 planets in tic T you enter pmode
if you lose 41 planets in tic T+1 you enter pmode

However, the planets you have gained in the last 48h do not count, I think the code uses the pop growth to find out which planet is older than 48h, so only those planets are counted by the pmode rule.

When you lose a planet which pop is 48h old (or older) then a pmode counter is increased by one, if this counter reaches X in the next 24h then you enter pmode.  Where X is half the planets you had 48 tics ago.

And I supose X is defined 48 tics after start of the round and takes a new value every 24 tics

so in tic 48 X=0.5 because you start with 1 planet

but in tic 72  X=3 because you had 6 planets the day after BoR  (notice X refers to 3 old planets, losing any other appart of 3 old planets will not increase the pmode counter)

and in tic 96 X=7 because you had 14 planets the 2nd day of the round  (notice X refers now to 7 old planets)

there goes my 2 cents

and so on...