Tax Credits Optimum Working Hours

[conf]

Is there an optimum number of working hours to claim tax credits? I currently work 30 hours and receive very little. (My partner works 30 hours also). I'm wondering if reducing my hours would be better?
I have tried the online calculator and it doesn't give accurate amounts based on my current situation, so I don't trust it's predictions either.

Only for working tax credits but with both of you working you are unlikely to qualify for working tax credits. if you have kids the cut off is around 17k I believe.

Thanks.