1. Take a big thick card and cut a hole into it, the size of your typical test strip.
2. Put the card onto your baseboard and position the hole where you want to have the test strip.
3. Tape the card down on one side, so you can lift it up like the page of a book.
4. Lift it and put your paper underneath.
5. expose the first test strip.
6. Now move the paper and do it again for a longer time.
It takes longer, but is very accurate and allows you to get the right time for exactly the spot on the print where it matters. If you like it, you can buy or make yourself a fixture which makes this all a lot easier.
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