You have a couple of choices, most involving creating more than one alert:
First choice, just set up F5_RAM_WARN, with the conditions you specify. It will be hard-coded across the board (all F5's will always warn at 75%, etc)
Second choice, use custom properties to set up per-node thresholds. You still need two alerts, but they're set up as "when RAM_Utilization >= <custom RAM WARN field>.
You can find more information on that here: https://www.youtube.com/watch?v=JIvXXX2wWaY
Third choice: NPM now supports per node values for warn and critical. Just go to edit properties for the node and set the value for that node (or nodes. You can do more than one at a time). Then your two alerts are "when RAM is in warning" or "when RAM is in critical".
Let me know if that gets you onto the right track.