Sorry, but I don't understand your question. Are you asking if a line could be made 1.4 of a particular font size? Methinks you'll need to explain it a lot better.
24px is what size? The correct answer that depends on the documents resolution. At 72DPI 24 pixels would print 1/3 inch at 300dpi the same 24pixels would print 2/25 of an inch.
You see pixel size is undefined till you know the resolution. DPI defines pixel size its pixel density. At 300DPI there are 9000 pixels per square inch each is 1/300"x1/300" ate 100 dpi there are 1000 pixels per square inch 1/100"x1/100" each
I'm looking at a web design. In CSS you can make a line height 1.4 based off the font size. But in Photoshop I have to manually do the calculation and type it in. If the font size is 24px, I need to type in 33.6px for leading.
Things are harder these days. In the old days displays were all low resolution 100 DPI or below, all also had a 4:3 aspect ratio. Displays had a had a small limited number of pixels typically 640x480 800x600 and 1024x768. So web pages were design to fit with a small number of pixels like 800x600 so a page would fit on a typical display. Then in 2001 IBM showed up with a 22.2" LCD with 9.2 mega pixels with a 204DPI resolution 16:10 Aspect ratio 3840x2400 pixels. The marketplace was not ready for that display normal PC images display very small at 204DPI and display adapters could not support 3840x2400 pixels.
In recent years mobile devices have come on the scene with high resolution and as many pixels ad PC displays. However PC pages displayed so small on these high resolution displays web pages had to be reformatted and scaled up in size to be readable on these devices. Next Apple came out with high resolution retina displays and again things needed to be scaled up in pixel size to be readable on these devices. Laptop and Tablets followed suit high resolution displays are becoming the norm for all computer devices except desktop machines.
Manufactures see profits in mobile devices and do not want to invest in desktop display. Its hard to understand for they are manufacturing the needed LCD panels but only packaging them in mobile devices, Top executives think they they know what best to increase profits to bad the don't have technical knowledge. Look at how IBM made Microsoft a household word.
CSS is part of your answer however these days you need to take into consideration the device the end user is using to display your web page and you need to serve up pages appropriate for their current browsing device they will have more then one. The one page for all devices days are gone.
Photoshop is a good Web tool however if web design is your job responsibility Photoshop should not be your main application.
I was wondering the same thing and stumbled upon a fairly decent solution. For leading or line-height, the "Auto" option in Photoshop actually uses a percentage. The default percentage of this option is 120%, BUT you can change this default to anything between 0 and 500. To do so, just select Justification in the popout menu of the Paragraph palette.
I do wish we could put a percentage value directly into the leading input field, but alas. Setting the default to 140% serves as a good starting point, and individual text boxes can be tweaked if necessary.
As I just recenlty discovered this technique, I don't know the all the ins and outs yet. Doing a quick test, it seems to be document specific and persists when reopening the file. I am going to try to use it more often and will report back.... if I remember....
Also, if web design is your job responsibility, Photoshop can very well be your main application.