Geophysics question - How do you compute the depth of a layer of bedrock, and how far should the geophones be spaced?

Hello everyone, Im studying for an exam and as far as i can find we have no examples to work with, just lecture notes.. bits and pieces here and there.

Heres the question

"A seismic survey is to be carried out to measure the approximate thickness of overburden, which is thought to be about 4m thick. Deduce the approximate distances from the shot point at which the geophones are to be placed to measure the depth of the overburden given that it has a velocity of 1.4 km/sec and overlies sandstone with a velocity of 2.4km/s and the fact you have 12 geophones to use"

I think you use this formula

h= t0V2V1

over

2(V2^2-V1^2)^1/2

Which I found in the refraction lecture slides.

But the answer i get when i compute it is h= t0(9.24km/s)

Which i dont know if thats what im looking for, i dont know how i can get t0 from it in order to compute accurately the hight of the layer.

And as for the how far apart the geophones should be placed.. the rule is twice as faras you want to see right? So 8-10m?