anything over 90% pure is good enough...
70% is a bit sketchy as theres a chance it will contain water or other additives that will leave residue.
but honestly ive used everything from tissue paper to dedicated tim remover.
i even used a hard eraser on my gpu (actually recommended for removing residue left by thermal tape).
but normally i just stick with clean and dry toilet tissue(none scented). if the paste isnt gritty.
it works well and i just keep rubbing till theres no black marks on the paper.
i know that this isnt pc and will likely get flamed for it but ive never had an issue or noticed any difference in thermal performance of the paste