Barclaycard Platinum Balance Transfer - Question on 90% limit

george.gp1

I recently received the above card with £5k limit. I understand that the maximum BT I could do is £4,500 with zero % int for 25 months. However before I do the BT next month (as I have 1 last month remaining on previous card) - I spent £500 on this Barclaycard (as it has 6 months 0 % on purchases)

The problem now is that my barclaycard site shows that I am only able to BT of £4050 as the 90% limit is calculated on available credit. So it looks like I have messed up myself because I spent on the card before the BT compared to if I had spent on it after the BT of £4,500.

Does anyone know if I am able to do a BT of £4,050 which leaves me with £450 of credit remaining (ignore fees) and then do another BT from the same card the next week of £405 (90% of £450)?

Thank you for your help.