MMC voltage rating

I have been pondering this for awhile, it gets more and more important
to ask:

Since I have, say 20 kV on the piggs secondary, the peak voltage would
be 28800 volts, right?
If I set the gaps to fire at this voltage, the cap will be charged to
this voltage, before the gaps fire.

But after this, doesn`t the primary swing opposite with around 80% of
this voltage?

So, shouldn`t We design the caps around a P-P voltage of (PIG P-P Vout *
1.8) in this case, 28800*1.8 = 51840 V.

Just wondring, Finn