Don't try to fit the centre dimensions to an arbitrary ratio; instead, consider the finished dimensions of your quilt, and how that will work for the quilt's purpose.
I like MadQuilter's suggestion of white/red/blue for the border order. If you use her border dimensions, you will have a 12" border all around. With a 43x70" centre, that will make your final quilt 67" x 94". To me, 94" is way too long for a throw (almost 8 feet!). In other discussions, people have said they like throws to be 65-80" long, so I would aim for that range. Subtract your border widths from a comfortable length to determine the length of your centre.
To figure out a good throw quilt size, find a blanket you like and sit on the couch with it. Is it too big? Too small? Too wide/narrow? Too long/short? Try starting with a blanket that is a bit big, and folding back the edges until you get a size you like. Then measure it, and aim for that size (+/- 6 inches) in your finished quilt.
Your quilt will be lovely no matter what dimensions you choose. Don't overthink it - enjoy the process of making it!