For household/modeling applications, eyeballing out the proper amounts is usually good enough. Just make sure you follow the instructions, and stir parts A & B really well so that you get a consistent mixture.
The stickiness could be an indiction that you're mixing them improperly (either wrong ratio or just not mixing enough). If you can rule that out, you may not have enough epoxy (small quantities take longer to cure than larger quantities). Applying heat may help, but I wouldn't recommend heating plastic models because they tend to melt.