Yeah, I have just been running the numbers too. I was finding it hard to square the increase in value of the shares (2x) with the cost to maintain your share holding (about 90% of the original). That seemed high. My numbers are (for round 3):
£50m post-money valuation
£4.8m cash injection
= 9.6% of company for that cash injection
If you dilute the original 0.0033 by this, you get 0.0029832. This is the new percentage ownership for the crowdfunded people (assuming £1k). If you then dilute that by the 25% dilution this round (25.287536) you end up with 0.02228828, i.e. 0.223 which is what @tom quotes here.
So, yeah, the answer to “why does it take 90% of the original investment to compensate for a 25% dilution, even though the shares now at 2x what you paid?” is “dilution in the interim round”.