It should be simple to calculate.
OP received 285 base points. Because RI stays should earn 5 points per USD, that means that Bonvoy thinks that the underlying room rate was $57. Unless the OP has included some extras in the 90 GBP figure, this is clearly incorrect.
However, there is no "online calculator" and you can be sure that Bonvoy's calculations will always be working in their favour, but roughly speaking they should be:
- Stripping out the 20% VAT from the 90 GBP --> approx. 75 GBP
- Converting the pre-tax rate to USD --> roughly $90-100 depending on the FX rate used
Therefore the base points should be in the range of 450-500.