I cannot say anything about other grain focusers except scoponet 20x. I normally place it on easel and focus without paper.
Focusing with paper has resulted in not so sharp print. :-( At-first I thought focus errors during shooting but I tried one more print by focusing without paper.
I do not know, scoponet might have built-in compensation for paper thickness(I know they may lead lots of questions...).
Enlarger: Durst 600 @f/8
Paper: Adox Vario Classic.
Print Size: 8x10 inch.