Assuming the QV Server Box is dedicated and not running other applications, as recommended. Then I would leave it it as is with the defaults, as it is designed to use RAM for massive caching which good.
Consuming 90% of RAM should not cause any issues. If it is then could share what the issues are ?
If you leave the QMC->Working Set Limit->High value set at 90%, that means that your QVS is allowed to consume 90% of the total RAM amount in your server. 256 * 0.9 = about 25GByte is left untouched.
All other QlikView services including any occasional Reload job run outside of this memory allotment. Let's say that your OS needs about 2GB of RAM, then your QlikView services and reload processes (QVB.exe) should do their magic in about 23GB of RAM. Is that enough? If not, you should lower your Working Set Limits so that more RAM is set aside for other services and reload tasks.
BTW you can monitor RAM consumption either live using Task Manager, or programmed for extended periods using Performance Monitor or another Windows resource monitoring tool.