Alberto, The way you are proposing is just fine. I use a method similar when entering ascii text numeric like this:
CASE $30,$31,$32,$33,$34,$35,$36,$37,$38,$39 '0,1,2,3,4,5,6,7,8,9
TARGETEMP = TARGETEMP * 10 'NUMERIC KEY JUST RELEASED SO ADD TO VALUE
TARGETEMP = TARGETEMP + (CHARACTER - $30)

That way you can enter as many numeric as possible. (up to 10 if using LONGS)