First.... if you're concerned about dps... fix the title from fps to dps.
Second.... couple simple rules...
1) Always make sure you have 2 diseases on the primary target.
2) Always make sure you have 2 diseases on the primary target.
3) See 1 and 2
Death Strike and Rune Strike are obvious. Last time I checked though, HS > BS for single target even. As a Dk tank, you should always see 3 diseases on target at all times. Icy touch + plague strike to establish frost fever and blood plague... and then scarlet plague from talents.
From there, it's a matter of wise rune management. Just don't lose sight of the fact that your primary job is still to hold threat and survive the consequences of that. If your raid is coming up short on dps.... it's good that you want to contribute.... but I doubt you're the "weak link".
No one tanks in a void.........