Your K/D ratio/percentage is calculated from the total number of kills and deaths tied to your character. I.e. If you want to achieve a certain ratio/percentage, you must constantly be achieving scores at/above that value. If you drop below it for one game or more, you must achieve a much higher value for a longer period of time, to get back to that certain ratio/percentage.
So, that round;
77 / 36 = 2.139 = 68%
Going by your ranking
Toulouse;
765 / 313 = 2.44 = 71%
To achieve a K/D pc of 75%, you'd need to have;
765 / 255 = 3 = 75%
or
939 / 313 = 3 = 75%
The K/D ratio is simply calculated by;
K / D
The K/D percentage is calculated using the following formulae;
K / ( K + D) * 100
K = Total kills
D = Total deaths
Basically, if you want to achieve a 75% K/D, you MUST get a 3 to 1 K/D ratio to begin with, then a 3 to 1 K/D or higher from then on to actually maintain/increase the K/D.
E.g.
You reset your K/D. You join a room and get 60 / 20 (3, 75%)
If you then join a room and get 25 / 10, your total K/D comes to;
85 / 30 (2.833, 73.91%, rounds to 74%) which is a drop of about 1%.
To get a 75% K/D again you must increase your 73.91% K/D by getting a 4 to 1 ratio or higher for several games, or simply get 5 kills no deaths.
You can work out more using these two links;
Calculator for ratio
Calculator for percentage