Your best course of action is to get hold of one or more proper 8-key flute fingering charts. Rick Wilson’s website has several facsimile period ones plus his own extensive (though not comprehensive) compilation chart. Pre-internet, I learnt 8-key using the chart in Otto Langey’s Boosey & Hawkes flute tutor book - 2nd hand copies can be found on eBay/Amazon periodically, or I believe a special-order scanned copy is available (at a price) from Boosey themselves (it is out of print). Several other systems are charted in Langey, and of course Rockstro gives extensive fingerings for many systems.
All that said, and these comments applying to antique flutes or accurate copies, not necessarily to modern makers’ tuning-modified flutes for ITM (where the C#, F# and E holes in particular are different from period originals), the main ones are:
Vent the Eb key for everything that does not require it to be closed (as for Bohm flute), save perhaps 2nd 8ve E (which is often sharp on 8-keyers if the key is vented).
In particular, the low E is much strengthened and brought up to pitch by venting the Eb - it will be flat and weak if you don’t.
For F#, vent one or other F natural key, according to which is more convenient in context (same principles as for F natural). Also be sure to vent Eb.
For (all-fingers-off) open C# at the top of the 1st 8ve, vent the C key - it will be flat and weak else. On many Pratten type flutes, C# at the top of the 2nd 8ve may be good thus too, but on most Rudall types you need to use the (very convenient) cross-fingering (oxx xoo,).
For high D (start 3rd 8ve) (oxx ooo,), vent Eb - it can make a very noticeable difference, as it can to C natural cross-fingerings in both 1st and 2nd 8ves
Those are the main, standard, expected-by-the-maker vented fingerings. Experimentation with them, especially using a tuner, will soon demonstrate their advantages in strength, intonation and tone-quality. Venting is important, even vital on sustained and accented notes but you can often miss it out in fast passage-work where to use it might be awkward and detract from the flow (though of course, with practice and familiarity you can get a great deal in).
I won’t go into 3rd 8ve fingerings here as they are, by definition, assisted harmonics of lower notes that all make use of venting (both of keyed and fingered holes) just to get them to sound, leave alone worrying about tone quality, strength and intonation. The “keep Eb open” rule goes out the window. In any case, most notes have several alternative possibilities which may work differently on different flutes and/or have special applications (cf Rockstro’s info and Wilson’s composite chart).