Measure your screen. We will just use the horizontal dimensions. Say it's 20 inches.
You want to make a 40x60 image. Long dimension of the print is 60 inches. So you want to magnify it by 60/20 or 300%
This assumes that 100% = full screen. Some software decrees that 100% means one campera pixel = 1 screen pixel. I think Photoshop and Lightroom both do this.
So instead: Click "fit to screen" Now the magnification will show up somewhere. THIS is the number you would multiply by 3.
Blowup ratio of screen = screen width in pixels/image width in pixels. This will be under 100%
Print ratio blowup = Printwidth / Screenwidth.
Lets do a walk through:
Load an image that is 4000 x 6000 pixels. My monitor is only 1500 pixels wide, so that even when I say "full screen" the magnification is 25% (1500/6000)
But my monitor is 20 inches wide. My print is 60 inches wide. So 60/20 = 3.
So if I set my screen to 75% I will see 1 cm of screen = 1 cm of print.
Note: Most of the time as images get bigger they are viewed from further away. An 8x10 printed a 150 dpi normally will have invisible dots. At 300dpi you need a hand lens. This is one reason why office laser printers are 300 dpi for normal work.
So do you need more pixels to print it at 40 x 60? Not really. The 40 x 60 will likely be viewed from 5-10 feet away. You can use bigger dots.
There are some exceptions to this:
A: If you have a mural that people are going to view from a distance, AND get close to to look at details, then the whole image has to be done at a size for the closer viewing distance. Murals in stairwells get this kind of scrutiny.
B: A photo that you are going to turn into a jigsaw puzzle falls into this category. You want each piece to have a sharp image when viewed from 7 inches away.