I have an idea. As you have found out that the flash is more consistent if it is set for relatively higher power rather than very low power. Perhaps, you could use density filter and meter the flash without the filter and use the filter for the actual exposure? You can set the flash power very high, measure with and without the filter to determine accurately the filter factor. Then dial the flash down to a level that when you put on the filter it would be right for your exposure. Finding very low level flash meter would be difficult.