I saw the following animation code at [Present] on every mod that has animations:

[Present]
if $auxTime % $speed == 0
    if $swapvar < $frameEnd
        $swapvar = $swapvar + 1
    else
        $swapvar = $frameStart
    endif
endif
post $auxTime=$auxTime+1

 Where $frameStart = 0 and $frameEnd is index of last animation.

It works, but if your ingame fps changes and is not fixed value then the animation changes its speed. Not a modder myself but after a quick investigation I wrote the solution which removes dependency on user's fps. 

Add the following under [Constants]:

global $animFps = 74 ; This is desired animation speed (in animation's frames per second) feel free to change it

Replace previous animation code under [Present] with this single line of code:

$swapvar = ( time // (1 / $animFps) ) % ($frameEnd + 1)

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