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Consider all the worst cases. Kvco is not constant with different frequency. Usually VCO's spec provides a plot of Kvco versus frequency. The plot of Kvco is usually a curve, so consider both ends and local maxima/minima, in the design of PLL.
u can assume a linear range for the VCO around the free running frequency,and in this range Kvco is linear and if u r using this VCO in a charge pump PLL u have to make sure that this range covers the frequency range u want from the PLL, if u satisfied this condition the PLL will always lock even if this range is not perfectly linear(but u may have problems in the settling time) of the PLL
designer's job is to make vco as linear as possible particularly in the middle. however sometimes the curve is no as ideal linear but when you use some tool to model it, you will be better off to use all the data let the simulator extract the kvco value. this will make more sense in reality.
You must test the region around your Free running frequency. at least 15 equidistant Vcand measure the output frequency. Then use linear regretion to obtain the slope is Kvco and the b is the Ffr.
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