Is wire-wrap spacing and consistency important in electromagnets?
With any ferrite core you should be able to easily saturate the core material, which will have a maximum flux density of about 400 gauss.  Additional current and/or turns will give you only marginal improvement in field strength (roughly the same increase as with an air-core coil).  In addition, the field from a ferrite rod diminishes quickly as you get farther from the end, because the flux direction quickly fans out.  Slight variances in distance will cause large variances in field strength.



You would be better off making a coil using a gapped c-core like the one shown below.  Laminated steel will provide higher flux density in the core by a factor of five or so.  More importantly, the field strength will be much mode consistent in the gap area, making you less prone to position-induced variations.


If you are stuck with ferrite rods, try using two rods in line with  end-to-end (N to S) with a gap between for your hall device. 



Good luck!