The company’s valuation after the investment is made.
Pre-money valuation + investment = post-money valuation.
Valuation of the company before the investment is made.
Post-money valuation - investment = pre-money valuation
If you need £500k to reach a milestone and on average a VC wants to own 20% of
the company (anything more is too dilutive for founders, anything less isn’t worth
VC’s time), then: