In the meanwhile (while you set up your user account on
ChemicalForums, make your post and await helpful replies there),
I'll offer brief comments on your queries.
Keeping the solution below 80% will avoid sample splatter of the
solution (containing the Ni2+ cations that you're carrying out
quantitative analysis on), which is one possible source of
error.
Chlorine has (relatively) high molar mass. If your precipitate
remains contaminated by Cl- ions (from the hydrochloric acid used
in the procedure), your calculations will be in error. Remember
you're basing your stiochiometric calculations (for this
quantitative analysis) on the assumption the dried precipitate
obtained is pure bis(dimethylglyoximato)nickel(II).
There are *many* (I can spot over a dozen) possible sources of
error, present in every stage of this experiment. I won't list them
out here, but you should be able to infer, deduce and elaborate on
at least a half-dozen of them, based on just the following readings
(read all of them, in the order given below).
http://www.kimia.um.edu.my/images/kimia/lab%20manual/level1/lab%20manual%20yr%201%20inorganic.pdf
http://www.cpachem.com/en/Clas/Ni-grav.asp
http://academic.cengage.com/resource_uploads/downloads/0030355230_17071.pdf
http://chemlab.truman.edu/CHEM222manual/pdf/nickelgrav.pdf
Lastly, a reminder & recommendation that all chemistry
discussions beyond 'A' levels, are best done on ChemicalForums.