While I’m not a huge fan of nitroglycerin paste, I understand it’s clinical usefulness. The ability of slapping on an inch of paste to relieve chest discomfort is certainly non-invasive and can achieve effective results.  With this simplicity, a degree of randomness exists with regard to the ability to titrate the dose.  If the desired clinical effect is not achieved, how much more can we apply safely? Conversely, if hypotension results, how long will the effects last after the paste is wiped off?
Though more invasive, IV nitroglycerin provides greater control and titratablility and one study suggests a dose conversion between the dosage forms. (Am J Crit Care. 1998 Mar;7(2):123-30)

The conversion from IV to PASTE is relatively straightforward. Apply the appropriate amount of PASTE, and then stop the infusion of nitroglycerin 30 minutes later. (see table below for conversions)
Converting from PASTE to IV is a little more difficult (and has not been studied).  After removal of the nitropaste, the duration of effect of nitroglycerin is anywhere from 2 hours to 12 hours. So titration to IV will be more difficult and require close attention. It would therefore make more sense to target the lower end of the conversion range. For example, if 1 inch was applied and the conversion range is 10-39 mcg/min, the IV rate should be started at 10 mcg/min about 1 hour after the paste is removed and subsequently titrated.
Of course if the decision to convert to IV was because the paste is not achieving the desired effect, the infusion could be started earlier, but still targeting the lower dose range.
5 mcg/min
10 – 39 mcg/min
40 – 59 mcg/min
60 – 100 mcg/min