Ric - you say you used to weigh the trout you caught. Did you keep records of length as well? If you did you could work out your own weight/length chart by interpolating (or extrapolating) from a few known examples. That way you would have a 'condition factor' that related to the trout in your particular water.
If you don't know how to calculate for larger or smaller specimens here's how:
The weight of a fish is proportional to the cube of its length; double the length and the weight increases eight times. In other words, if an 8-inch trout weighs 4 oz in one water; a sixteen inch trout from the same water ought to weigh 2lbs.
To work out a range of lengths and weights from a single known example*, divide the proposed length by the known length (in this case 8") cube the result and multiply this by the known weight. So, for a 6" trout you have 6 divided by 8 which is 0.75; the cube of which is 0.42; multiplying this by the known weight (4oz) gives a weight of approx 1.7oz.
For a proposed length of 19 inches it will be 19/8 = 2.38. Cube this gives 13.4. 13.4 x 4 = 53.6 ounces, or 3lb 5oz.
*To get a better idea of the general condition of 'your' trout it would be prudent to record weight and length of several fish and work out a condition factor for each. Just cube the length and divide by the weight.
Suppose you have two trout of 8 inches, one weighing 4oz and one weighing 5oz; the first one has a factor of (8 x 8 x 8 = 512/4) = 128, whereas the second is (8x.... = 512/5) = 102.4. You can take an average from these of 115.2 and work out a notional trout of eight inches thus: length cubed divided by condition factor equals weight (8 x 8 x 8)/115.2 = 4.4oz.
This principle works for most species - Mona's Scale is based on it, which is a pretty accurate guide for the weights of pike (except when carrying roe, as the large females usually are in the last couple of months of the season). However, it doesn't apply too well to carp as, even in an individual water, their proportions can vary so much. For carp it is better to use George Sharman's system, although this is more complex.