New to the game, 1st draft, 24 pitchers and 1 DH.
My 1st round pick is linked below

http://whatifsports.com/HBD/Pages/Popups/PlayerProfile.aspx?pid=6358528

Potential durability of 12.
Potential Stamina of 72.

I had thought I had a good SP. but in posting in my coaches content I was told he would end up being a good relief pitcher.

Is a good rule of thumb for SP to have higher DUR/ST combo rating?

Coaches corner. Freakin autocorrect.

I would say that if you want an SP to pitch to his stamina limit every 5th day, he has to have a DUR of at least 20. I have a few in the minors under 20 and use them as long relievers, but they need rest after every game.

Weirdest pitcher I have goes 6 / 58. Even as a one-inning setup man, he drops to 90% after every appearance. Never pitches two games in a row.

what would be a good minimum threshold for a SP in regards to Durability and Stamina (for projected ratings)

Depending on how many innings you want to get out of him, I'd say my (near absolute) minimum threshold is somewhere around 15/65.  That's very low and of course there are always exceptions, but if I had a stud at 15/65 I'd definitely consider starting him.  Ideally you'd have a 25/75 or close or better.  

Also having a solid bullpen with guys capable of pitching a couple of innings fairly often allows you to deploy lower dur/sta type guys in the rotation.   

I have no problem using a guy with 40s stamina as a SP.

Kyle Anderson (R)		36 (62)	28	79	100	57	69	73	39	81	86	79	61	60	39	$1.6M
Jimmy Powell (L)		100	29	92	70	90	96	87	72	64	85	76	64	64	44	$6.4M
Andres Jimenez (R)		100	30	88	82	49	73	89	39	75	74	60	45	39	0	$5.8M
Andres Castilla (R)		100	26	77	44	81	71	80	98	83	79	77	46	57	0	$360K
Will Mahoney (R)		100	20	99	56	80	89	88	90	31	73	76	53	46	27	$343K

But a durability below 20 is problematic because the recovery is too slow.