I've used the system. Overall, fairly effective.

In building the fumigator, I couldn't find the felt they suggested, so just
used a few layers of blue shop towels.

Using the WVa fumigator, I tried their prescribed strength (80-110 ml of
50% formic) and I burned a few colonies. They all fled from the hive.

Another member that I met through Bee-L suggested the following strengths:
For all - temps 60-94 deg. F
2 Deeps - 85-95 ml 50% formic
1.5 Deeps - 75-80 ml 50% formic
1 deep - 60-65 ml 50% formic
All have 15 ml HoneyBHealthy added

Mite counts using the above method went from:
- Hive 1 - 17 mites per 100 ml sample on 5/28 to 2 mites per 100 ml sample
on 6/3
- Hive 2 - 13 mites per 100 ml sample on 5/28 to 3 mites per 100 ml sample
on 6/3
- Hive 3 - 8 mites per 100 ml sample on 5/28 to 2 mites per 100 ml sample
on 6/3

I went with the higher end of the above range, and some of the bees
evacuated, which I think impacted my efficacy rates. Tough to tell though.

I attempted to repeat the results in September over more hives.
Unfortunately, I didn't have the felt the WVa people recommended, so I used
several layers of blue shop towels. Over the three month time period I
think the shop towels broke down some, and didn't hold the liquid. I plan
on using the actual felt next year and seeing how it goes.

I hope that helps.

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