UI element width association with screen size

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Hello, I writting table UI element and I have one of the columns where content string can have maximum 10 character string length. Then I think, about percental element width, I realise that there might be several problems with it.

If I design for smallest screen possible, the width of string "1234567890" may be around 50 pixels and take 40% of screen width (for 128x96 screens). When displayed on screen with 640px width, the same element will take 290 px space, but the "1234567890" will take not more that 200px using the default font. Thus presious 90px are wasted.
And if I design for the big enough screen and hard-code it's percentage, the string may just not fit in the dedicated space on small screens..

Are there any cons of percentage based element widths or other approaches to UI element width calculations ?

I'm looking forward to your comments and ideas!
Thank you!

[Tiger79]

u could play with Font sizes, so depending form the resolution changing the Font size... smaal screens u could setup to use smaller Font so that more data fits in one single column (line)...