Missions & Complexes

Basically you want to have covops 4 and all the astrometrics skills at 4 if youre gonna do probing regularly.
Absolute minimum I'd say you want is covops 3, astro 4, astro support 3.

To get the hardest sites in low sec, you need 90 minimum. I tried with 80 scan strength and a perfect tetrahedron shape and could not get the signature scanned out (combat ladar with named faction pirate -- hardest low sec site to scan). Once I got 90 scan res I was able to scan it down (admittedly it was still some trouble to get it 100%, took some playing around with the tetrahedron size)

I now scan with 110 scan strength and have little problems getting any site to 100%, only the [hard] combat ladars still require me to go to 0.25 AU (though no special configuration required at all). Can tell what most signatures are by 4 AU.

Im sure there will be somone arguing like a madman but i managed to pinpoint mags, radars, ladars and combat sites in low sec, C1 and C5 wormholes (didnt test other places) in all of them without having rangefinding and pinpointing at all. However i did have amarr frig 5 and sisters launcher with sisters probes in magnate.

So far from my experience i can tell you that to actually get 100% you dont have to have pretty much any support skills at all. Just basic crap should be fine in theory.

However there are 2 catches:

One is that with low skills you need to have near perfect placement of probes at minimum and/or near minimum AU settings to get 100% (i.e. warpable) hit. This can take literally seven frustrations and advanced irl anger management trained to 5 to do.

Besides frustration there is time constrain. These days with good supports, cov ops, sisters or T2 gear and low PPH + PPF implants (if you feel silly rich why not even low grade virtue set, i dont do it) it takes far far far less effort to get 100% hit and some sites can be found even with 2 AU settings and the time it takes me less can be measured in multiples.

Also pro tips and very common mistakes people do:

When overlapping speheres it is better for the signature to "touch" the edge of spehere rather than inside of bubble.

4 probes per sig is the crappiest number for probes. Easiest multiple probes is 7 - north, west, east, south, up, down +middle. More probes allow bigger offsetting on probe accuracy.

Deviation modifier from gear/skills is actually the factor of how much you can be "wrong" of your placement of bubbles. This is retardedly underestimated by majority of community in how fast you can scan your sig down and people use probe strength as if it was stat made by god himself.

Combat sigs (pos structures, ships etc. with combat probes) take a LOT less strength with worse placement of bubbles to pinpoint.
---
My theory that i can not prove which means "gut feeling true facts" is that the strength and deviaton required is based on type of site not system you probe in. I only have this feeling as i tend to pinpoint crappy gravs and wormholes fastest compared to high end radar/mags regardless of which class wh or wheter in low/null im in.

Edit: Once i am able to confirm or deny a few theories i have regarding probing im considering making a guide out of it.

I think you just discovered what people are doing with deep space probes (and normal ones too): lookee here

And yeah deviation is nice, but you sorta need probe strength first to be able to lock sites.

Does EFT display scan strength and deviation on fits?

EFT does as well. Load your skills, load the right probe(s) in the launcher, right click, show charge info. Scan strength, as modified, will list correctly.

Oh, and I've never known the DSP guides to be off by more than .01 and that's likely due to me inputting my scan strength from memory rather than checking.

Because you had the luck (or bad luck) that you never scanned in a system with weak signatures. (They are rarer anyway.) For sure you cannot scan down all such weak signatures in an ship without bonus and with low skills.