The second shows a comparison of three different values in that field. The far left material is set up the way that you have yours, in which I multiply by .2. The second has the multiply value changed to 1 to simulate no changes being made to the emmisive light value. The far right material has the multiply value set at 15. You can see that by comparison, multiplying the value by .2 (or 2/10 in fraction form) actually divides the emissive output making the material appear less bright than the original value. Using a positive number for this value will give the results that you are looking for.