Actually, the only time that the old width/heigth settings made any sense was if your thumb size was exacly 100. Because what really happens was that these values were used as proportions to the thumb size. Now if you think this is more intuitive than what we have now.....
You can, of course, still get your 3/4 ratio using the percentages presuming all your images are the same size. (If they were not the same size you could not have done this at all previously)