Tapping the engine bay fuse box

I recently bought myself a snooper radar/laser detector that is designed to be installed between the radiator and the grille and wired to a (preferably switched) 12v supply. It then links via Bluetooth to an app on my iPhone.

Unfortunately Halfords (who I bought it from) weren't much use when it came to installation, but I figured I might try to do it myself if I can. I've managed to work out how to remove the grille and think I should be able to physically mount it ok, but I am a bit of a novice when it comes to car electrics. From what I can gather, tapping into the fuse box is probably my best bet if I want a switched supply - but I'm just wondering if anyone has had any experience doing so with the Evoque? From looking at the manual (and the actual fuse box in the engine bay) it seems there are some "spare" fuse slots - but are these likely to have any power if I use a fuse tap, or am I best off tapping an existing fuse? Also will they be switched with the ignition or live all the time - or does that vary from fuse to fuse?

I've got myself a mini fuse tap from ebay (the engine bay seems to use the "mini" size rather than standard) but before I start playing around I thought I'd ask here in case anyone had any previous experience and could help me avoid doing anything daft!

If all else fails I'll try to get it professionally installed, or perhaps try to exchange it for a more traditional dash-based model, but thought it wouldn't hurt to at least look into doing it myself.

Thanks in advance!