Exchange 2010 limit RAM Usage

Dear experts,

I know this question has been asked a few times before. However, I'd like clarification before I go ahead and make changes. I have an exchange 2010 SP3 with approx 40 users. This is running on a Server 2008 R2 with 20GB of RAM. The issue is the usual one of store.exe taking up all of the available RAM and then the machine becomes very slow.

Have been looking at the guide below;

https://social.technet.microsoft.com/forums/exchange/en-US/00851cd4-bc74-4d94-95b7-8982c5a93380/limit-memory-usage-on-exchange-store

I would like to limit the store usage - however, I'm not sure how to work out what the upper limit of the store RAM usage should be - hence the question - how is it determined what the store upper limit should be so that I achieve a balance between limiting store.exe and also gaining acceptable exchange performance. I'd rather have a guide or other calculation rather than simply guess what the upper limits should be.

Thanks in advance.