DWThomas: Yes it was with a pinhole camera, made out of a cardboard box. I metered with my normal lightmeter @iso5 f/22 so I would now how the light was, the reading was 4 seconds.

I used Ilford Multigrade RC IV deluxe as negative. ( developed in Agfa Neutol 1+9 for 1 minute)
Then I started out testing exposures, first 1 minute exposure, then 2 minute, then 4 minute and then 8 minutes. 1 and 2 minute was way too dark, 4 minutes I was getting there and 8 minutes was a maybe a minute too much. So 6 minutes 40 seconds like you suggest sounds perfect.

Do I understand you correct that if I use this method each time for this pinholecamera I can just multiply my lightmeter reading @iso5 f/22 with 100?