@tommy5dollar
Very interesting approach, but would that scale well when we have £10kk pledged and only 1/4th will be in?
I don’t remember exact numbers of first investors, so I’ll take it was 1700 people.
We have £10kk pledged. 2,5kk is crowdfunding.
Version 1
Assuming that old investors will invest max, that leaves 2,5kk - 1,7kk = 720k.
With 10kk pledged, and average oscillating around £270-280, we are looking that 35,000(ish) people want to invest (on that average). 720k divided by 35,000ish people would allow to invest max £20 per person.
With certain percent of people investing £10, it wouldn’t technically allow to invest much more over £20, definitely not reaching sums like £35.
Version 2
Lets assume that old investors will invest on average £500, I made it higher than average pledge now because it seems reasonable. 
Old investors will take then 850k out of 2,5kk, leaving sum of 1650000.
With 35,000 people willing to invest, average investment could be around £47. Again, with some people going for £10, £20 etc pledges, it leaves some room for other people to higher, but probably unlikely to go over £100.
Version two feels better, but the outcome is solely based on fact that old investors invest gently. 
Version one is not far from “we allow everyone invest £10 only”.
However, both versions guarantee investment.
Devil’s advocate now, but wouldn’t having 35k (+1,7k from old round) investors actually degrade the value of being investor? Well, not if Monzo reaches target of billion users, but before this happens, in current proportions - 1/4th or 1/5th of all Monzo users is an investor? 