Try doing the test strip this way. expose the whole sheet for 5 seconds, then cover section 1 and expose for a further 5 seconds, again cove the next section and expose for a further 5 seconds, continue untill you have sections up to 30 seconds, that way you will clearly be able to see each section and be sure you are looking at 5,10,15, 20,25 and 30 seconds, and be genouroes with the wedges.
Richard