I use the setters with keyboard nudge a ton for quick and accurate layouts. It is important when making fluid layouts using percentages to have the ability to nudge decimal values (one or two decimal spaces). Many design apps and Chromes > Inspect Element allow you to nudge with option + arrow up or down to increment in tenths .n values. This would be a tremendous boost to speed when setting fluid percentages.
I can provide examples (screencasts) of advanced methods that apps use for value setters if it would be helpful. The Foundry > Modo has about the best usability I have seen in any app, with scroll wheel implementation on value setters.