Gordon, I also have a Flexaret (mine is Flexaret V), and mine has a filter list and exposure corrections on the back, for "Ortho", "Ortho-Pan" and "Pan" films (different emulsion which, I think, are not available any more).
Anyway, from the numbers there, I guess the filter factors and corrections would be as follows:
(Filter > Factor > Exp. correction, in f-stops)
G1 > 1,5X > -0,5 (Light yellow)
G2 > 2x > -1 (Medium yellow)
G3 > 3x > -1,5 ("Strong"/dark yellow)
B1 > 2x > -0,5 (Blue)
R1 > 6x > -2,5 (Red)
The first number is correction factor, and the second number indicates correction in f-stops, e.g. for G1 (light yellow) filter, if the indicated exposure without filter is e.g. 1/60 at f 5.6, you need to open up half a stop, so the exposure would be 1/60 at f 4.5 (between f5.6 and f4).
The filter designations are from German language - using probably old Zeiss markings or something, where "G" means "Gelb" (for yellow), "B" is "Blau" (for blue), and "R" is "Rot" (for red).
You could also find filters designated as GGR (Gelb-Grün, i.e. yellow-green).
As for the usage of filters, I'm afraid that's a material for a full-sized book, and you should look it up also on the Web, if you need some more in-depth info.
I can only provide a short guide - yellows will darken the sky a bit (depending on grade - G1, G2 and G3, where G3 yields the strongest effect). Generally recommended for use with B&W film outdoors.
Blue (B1) will lighten blue subjects and increase bluish haze.
Red (R1) will lighten red and yellow subjects, darken blue water and blue skies dramatically.
In general, filters render their own color lighter (i.e. yellow filter will lighten yellow leaves, red filter will render a red apple very light, etc.), and darken their complementary color, i.e. make it darker.
This is just a very general interpretation, and I recommend some Web searches - I'm sure you'll find very extensive explanations.
In general, if you're doing B&W shooting outdoors in clear weather with blue skies, a yellow filter is a must - otherwise the blue skies will be washed out (white). Personally, for emphasizing white clouds on the blue sky, I often use orange filter - it's somewhere between yellow and red filter in effect.
Attached is the photo of the filter table on the back of my Flexaret.
Hope this helps a bit.
Merry Christmas!!!