Reboot does not take 30% of the battery, since that would require the tablet to draw much more current than the system can actually stand. The system at maximum cannot draw more than something like 3W, and since the battery has about 5Wh, it takes about half an hour (at full draw) to waste 30%.

I saw a similar effect many times - battery appears half full, or even
3/4 bars, then I reboot and it has one bar, or none at all.

What I suspect happens is this: Estimating battery charge by voltage is not very accurate, as it depends on temperature and current draw, among other factors. Since Nokia don't want the values the tablet display to jump all the time (like reporting there are 7 days of standby and then after a few minutes
of use but no charge reporting 9 days), they don't display the raw value read from hardware, but apply some smoothing algorithm, which obviously depends on past readings. After a reboot there are no past readings. so the same (or similar) readings from hardware translate to different values reported to the user.