It's easy to determine the optimum exposure time for a system even if you don't have an exposure calculator such as the Stouffer strip. Commercial exposure calculators are neutral density filters that allow specific percentages of the light to pass through so that you get a range of exposures from one experiment. This can be achieved in a less elegant but more practical method.
Standardise all your variables:
Thickness of emulsion
Emulsion type
Exposure difference
Temperature
Take a coated screen and cover all but about an inch with a piece of card, expose for a minute.
You may have a good idea what the exposure should be, you can alter the range given in this tutorial to suit your system.
Move the card over an inch and expose for another minute:
After a few minutes you will have a screen with a range of exposure on it:
When washed out it may look something like this:
1&2" have washed out, 3&4" are underexposed, 5" is OK, 6 is probably safe.
If you haven't got a reasonable range, just go back and do it again.
Your light source will deteriorate over time so you should repeat at regular intervals and if signs of underexposure occur.
It will be a good idea to have a positive with some text on to check for overexposure, when the open areas are filling-in you are overexposing.
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