I know I have read somewhere that the skill is at it’s max % when your shield is completely down, but has anyone determined the actual formula the skill uses to determine the chance for free shots?
I am also wondering if the Rough Rider is worth using just for Inconceivable as I don’t go far enough into Brawn for OoBG, or if I should use something else like the Blockade.
Made a minor edit to your post in light of the FORUM RULES relating to data mining.
I have a feeling that this is a topic which has been addressed on the forums before - maybe @DeputyChuck or @Sljm know about that?
Let’s edit a word to make no sense, good call.
This was as much as I can remember seeing :
There’s a link to an old forum post that I can’t get to work in the archive.
Chuck alluded to Sljm having done some work on that years ago.
I don’t mind, I guess I missed that part of the rules, other games I have been active on the forums were less strict on that specific issue.
I did have to edit your edit because of a misspelling.
The rules were updated earlier this year, in order to clarify a few things. Anyway, I know there was some investigating on this issue, but I’m not sure how far it went. Just going to ping some of the other suspects (@Piemanlee, @Jefe)
See 3 posts up 
Actually, can you see if you can get this to work in the archive. I’m on my phone and can’t get to a computer for a while.
Edit : got it!
…except it doesn’t really help 
The rate is max when either your shield or your hp is 0. There may be an equation for varying levels like half and half but it’s neither practical nor really useful to find that. I did do very large samples with Rough Rider to confirm the rate is 50% (RIP credits)
Rough rider maxes it unless your class mod reduces shield capacity to less than zero, but those mods all kinda blow anyway.
Any downed shield does the same thing, e.g. Flame of the Firehawk.
Starting the fight at like 1 hp with a full shield would also (nearly) max it, but… Why?
I guess the secondary question the OP had was : is it worthwhile for Inconceivable to use a RR for a non-Brawn build (especially since it lends itself to health stacking taking any potential low health exploits off the table)?
Personally I use a RR most of the time so it seems (…wait for it…) inconceivable to use anything else.
You keep using that word… 
Eh, I’m not a good one to answer that, I use FotF 100% of the time. When Pimp glitching raids, even. Free heals, y’all.
Personally, my answer to that is yes. The RR is always a good shield for Sal.
I usually use an Evo or a Neo, but they are down pretty often too since their capacity is really low with the parts I chose (maxing out extra health) so they kinda play like an RR.
If you don’t put a single point in brawn, you could make a case for a different shield, but at this point, the FotF becomes a much better choice anyway.