[Aeraaa]

As i said, I know that 20 pounder can penetrate a bit over 300mm of RHA without slope. The problem lies in that its performance drops a lot against sloped armor (especially against highly sloped armor like the T54s front hull), so no, a simple x/cosθ will not give the effective armor thickness of the target. The only rounds that follow the above simple equation relatively close are HEAT rounds, but none of the KE rounds do(modern APFSDS are better in this and some like DU penetrators are actually better against sloped armor than vs thick unsloped one, hence the boxy design of modern MBTs). Anyway, in KE rounds, things like thickness/diameter ratio, armor quality projectile speed etc give very different results that vary from round to round and in the case of APDS rounds a slab of armor of 100mm RHA sloped at 60 degrees from vertical has 3.5 effectiveness against the incoming round, which makes the T54s hull a very hard nut to crack even at point blank range (source: WW2 ballistics armor and gunnery by Lorrin R. Bird and Robert D. Livingston pages 30-31, although it speaks mainly about 17 pounder, 20 pounder wasnt much different).

Btw DRG the link you gave is from warthunder and after many updates this games handles sloped armor differently. 20 pounder now penetrates 80mm @60 from vertical, compared to the 100mm @60 needed to defeat the T54's front hull.