the process is the same for n810 or n800 give or take a couple minute differences.
reboots do take a chunk out of the charge each time...yes.
Concerned about the battery condition?
Unless the battery has less than a charge capacity of 2 hours...
you should be able to get past the literal 2 minute "flashing stage" where being unplugged from a power source is necessary.
Then plug it back in and carry on with getting it snappy again.

Suggestion is.
Charge it.
Literally to the point the device says "battery full"
unplug the power cable.
Immediately if not sooner (don't want the battery now sitting for days...especially if there is a prob with it keeping a long term charge..)
Prepare your pc.
install libusb properly on the win7
install the nokia tablet update wizard properly on the win7.
connect the device and attempt flashing.
If there is a problem it will be with an omitted step in preparing the pc properly.

Essentially ...follow the walkthroughs or vid tutorials