Here is how a particular setup is likely to perform. The floating line is a WindCutter 10/11/12 (RIO) with the front floating tip removed and replaced with a Type 8, 166 grain sink tip, which comes packaged with the line. The leader is 3' of 15 lb test monofilament. The fly is an unweighted tube fly (2.5"). Water velocity is about half of normal walking speed.
At hangdown, the Type 8 sink tip has pulled the end of the floating line about 1 foot underwater. This plus the rest of the sink tip results in a fly depth of about 4.5 feet. In contrast, an Intermediate sink tip with the same setup drops the fly to about 1.5'. Even though both sink tips have the same length and grain weight, the Intermediate tip has a larger diameter. For this reason, it weighs less underwater and is lifted more by the water flowing underneath it -- resulting in less depth.
The chart below shows simulation results for a wide range of common sink tips. This chart contains the same information as published in "Ruling Your Fly Depth," Fly Fisherman Magazine, December, 2007 (click for free download). The butt of each sink tip below is attached to the same standard set-up: a 26ft 650 grain floating Skagit head, 3 ft of 15 lb monofilament leader, and a small unweighted fly. Click here for a BIG PRINTABLE VERSION, or click on the chart itself.
The sink tips in the chart below have little or no taper. Click here for a comparison of tapered sink tips.