Does Z travel make sense as a number?

[JAZZCNC]

Many reasons for a high Z-axis but no right or wrong for either, it's all down to usage.
For instance, I recently built a machine with a lifting gantry that had 400mm of Z-axis travel but was never going to cut deeper than 2 or 3mm, the height was needed because of the part size.

I've built similar machines with high gantry's and Z-axis because they wanted to drill holes thru 10" beams and machine scarf joints, pockets, etc for log cabins, yet still, they wanted to engrave slate which was only 5mm thick.!
In cases like this, you just have to accept that there is a price to pay for a large Z-axis. Raising the bed is an option for thin material but it soon gets tiresome having to surface the bed every time you want to machine something thin.

Also, regards pocketing 150mm deep then you better have deep pockets, have you seen the price of tools over 100mm in length.?

Drills are probably the longest thing you'll stick in a spindle and a good reason for a decent amount of Z-axis travel/clearance.