Spider, I noticed you are erasing the formula and typing your required size in the original box. This can sometimes lead to problems if you have extensively paremitised your model. A more stable way is to leave the original formula which is contained within the inverted commas '..........' and add your amendment as a mathmatical equation onto the end e.g.
'Sheet Metal Parameter.1\Bend Radius' + .25in or if you are using metric
'Sheet Metal Parameter.1\Bend Radius' + 6.3mm
Actually any mathematical sign can be added. However, remember to leave a space after the last ' before you start adding. This method not only keeps the model robust it also allows the re-introduction of the original value by simply deleting the additions.