To calculate how far you can move after you stand you subtract how much movement you've taken. If your speed was 30 at the start of your turn and you did not yet invoke feline agility it took 15 feet of movement to stand.
Your movement was 30. Your speed was normal.
If you do not change your speed the amount you can move the rest of your turn is 30 - 15 for 15.
agree
If you invoke feline agility after you stand your speed changes from 30 to 60. To calculate how far you can now move on your turn it's 60 (your speed) - 15 (the amount of movement already moved) for 45 feet.
Wrong. Your speed is double the normal speed after invoking FA and therefore doubles the remaining movement from 15 to 30
This works the other way too. If you had used feline agility at the start of your turn to close in on an enemy caster and you move 40 feet which triggers the caster's readied Slow spell you may need to recalculate how much further you can move. If you succeed (you are not slowed) you had 60 (speed) - 40 (movement taken) meaning you could still move 20 more feet. If you fail your save (speed halved) it's 30 (speed) - 40 (movement taken) which is less than 0 you cannot move any further.
The equation is partially correct, you had double speed for 60 movement, if you succeed your sve you can go for another 20, but if if you get slowed after 40 you which returns your speed to normal you got 10 left.
No addition is used in the equation. I think it's a reasonable and clear way to handle mixed movement speeds. Unless I totally misunderstood what you were saying of course.
Yeah you should not but you are only adding and substracting. Your speed is doubled means your movement gets doubled.
Your normal speed,
whatever it is alows you to travel 30 ft in 1 round. If one round would b 10 seconds then you would move with 3ft/second which is an actual speed. If a round would last six seconds then you would move with a speed of 5 ft / second.
Since there seems to be no definition on wether a round takes 5 or ten seconds (which both would be realistic values),
you do not know the exact value of your speed.
What you do know is whether you are going at your normal speed, or double speed (hit by haste spell or invoked FA) or half your speed (hit by a slow spell).
The prone stand up takes
50% (15) away from your movement as written, but does
not alter your speed. The remaining movement distance is 15ft if your normal movement distance is 30ft at your normal speed. You may double that distance with a speed buff like FA or haste cast at that moment to be 2x 15ft = 30 ft.
If you had a speed buff by FA at the start of the round your theoretical movement distance would be 2x 30ft (double your normal movement) =60ft because of that buff, but the prone takes away a linear
50% of the movement, not caring if your speed had been doubled, so this would be 30ft at that point, leaving you with 30 ft of movement
By chance the 50% and the doubling due to FA eliminate each other, the numbers would not be so straight if
e.g. the stand up would take away only 10% of your movement, but still make no difference mathematically:
If it were like that, with FA upfront you would lose 6 ft of movement and could move another 54
With FA after standing up you would lose 3 ft and have 27 left which you could double now to be 54
You see, it really doesnt matter it is a linear relation, a 1st order function
If the stand up from prone would consume 100% of your movement then it also would not matter. Because FA does not work like a bonus action dash, but rather like a haste buff. You could stand up, but not move an inch in horizontal direction, no matter if used FA before or after.
In a normal round it does matter a lot.
FA upfront: If you moved (at double your speed) for 6 ft and got hit by a slow you could go for another 27.
No FA upfront but you moved (at your normal speed) for 6 ft got hit by slow have 9ft remaining and you could now only double that one to 18 only. So this clearly seems to be disadvantagous in this case, interesting, huh?
The difference is that with the prone stand up it takes away 50% no matter if your movement distance would have been 10 ft or 100 ft in that round. so if you double up speed before you got double your normal distance left, If you double up after you can double the remaining half, which is the same.