First of all if you are using a web aplication you can simply assign the same transparency value using the alpha value.
And now, for the 80% percentage simply use an alpha value
color: rgba (111,162,152,.8);
If you want a solid color take a look at this post: How to make a given color a bit darker or lighter?
The diference is that you need to define this diference between the value and the absolute white. This will give you tour starting 100% and a margin to lower its value.
For example R255G0B0 has a maximum diference to white of 255, the diference between 100% and 80% is 20% so the value is 255x(20/100) = 51
The 80% red would be R255G51B51
In your case
- R 255-111 = 144x(20/100) Plus the original 111 = 139.8
- G 255-162 = 93x(20/100) Plus the original 162 = 180.6
- B 255-152 = 103x(20/100) Plus the original 152 = 172.6
The general formula would be
(255-x) [(100-%)/100]+x = y
- x= the original value
- % the percentage used
Of course this will only work asumming the background color is white.
(Dam. I love math!)