It'll be most effective if you use fluid lecithin, and added at the beginning of your conch cycle.
There are a number of study researches, which suggests that the maximum supplement lecithin is 0.4%. For example: 1 Kg of chocolate can be added to 4 grams of lecithin. More will lead to a gradual increase in viscosity.
Practical recommendations. Lecithin mix with an equal amount of cocoa butter and add the chocolate for 1-2 hours before the end of work. Formerly adding lecithin is not recommended, as lecithin under the influence of temperature and mechanical stress may collapse and lose their properties.
As with almost anything, i'm afraid it's not quite as straightforward as a 0.4% max. While that can certainly be true for some instances, the max amount depends on a number of variables including PE/PC ratios (the actual amount of the functional components found in lecithin - not all lecithin is created equal!), the geometry of the particles one is trying to coat, the amount of moisture present (and if it's present in the form of waters of crystallization or mono-layers or free moisture), the starting viscosity, the starting fat % and the extent to which the fat that is already present has coated the solids, the temperature, the ambient relative humidity, the amount 'work' input into the mass (which is a function of the design of the conche and some mixture of the above elements), etc, etc, etc
For the average individual here, almost none of those elements are known or controllable, and the scale at which they're working is relatively small. The equipment with which they're working is going to require them to utilize more lecithin than is technically necessary were they to have the ability to work with other types of equipment where the work input allows for more control (ie most folks here are adding lecithin to a chocolate that's already either semi-fluid or clay-like in consistency), and as such they are not inputting anywhere near the optimal amount of energy/work to obtain a controllable, optimized conching envelop. Under those conditions, the bridging which occurs with too much lecithin usage, generally doesn't start to appear until 0.9-1.0% usage levels; i recommend starting at 0.3-0.5% and incrementally (0.1%) add more until either the max viscosity reduction is obtained, or the target viscosity is reached - which ever comes first.
I would still recommend to add 0.4% to 0.5% as a maximum. And if the target viscosity is reached, then further added cocoa butter. Too much lecithin can also lead to problems with tempering and crystallization.