Since we don't know your network or config, it's impossible to know if this is normal for you...
"Normal" memory usage always varies by:
1) Physical RAM in your FortiWeb model
2) Number of policies configured and enabled
3) Buffer sizes (caching, antivirus scanning, protocol constraints, input enforcement, etc.)
4) Your normal traffic volume, plus some allowance for spikes
If your FortiWeb handles your spikes well, then maybe there's no problem. In that case, you can just increase the alert threshold to, say, 74%.
If it doesn't, try disabling unnecessary signatures and policies, and (if possible) decreasing buffer sizes. There's a section in the Admin. Guide about performance tuning. And if you look at the index under "performance", lots of related settings flagged throughout the guide.
If none of that works for you, you can get a model with more RAM.