Generally speaking there is not much need to go above 0.5% acetic or formic acid for most ANP applications. If Trifluoroacetic acid (TFA) is used such as in biological applications, the usual limit is about 0.1%. TFA is generally not recommended for LCMS because of the ion suppression it can cause.
Below are charts showing that low levels of formic acid are often better than higher levels. This is typical of ANP methods.