iphone storage full, can't back up to computer

I have a 64gB iphone 8plus with full storage (63.8gb), but 40gb of this is "other". I have tried all the usual ways to delete storage, but can't get rid of the "other". As a last resort, I want to back up my iphone to my PC and restore the phone to new, hopefully getting rid of the "other". When I connect the iphone to the PC and open itunes, it says there is not enough (iphone) storage to back up the iphone.

How much free storage does an iphone need to back up to a PC?

Thanks.