For the benefit of you and others asking for how many marks are needed for different grades, no one knows until grade boundaries are published in August.
I suggest you familiarise yourself with the awarding grades process: how the marks you receive in your exams are converted to a UMS score which determines which grade you receive.
This depends on the raw mark grade boundaries that are decided for each unit which then convert to a UMS score (which determines the overall grade: 90% UMS is an A*, 80% UMS is an A, 70% UMS is a C etc.) Past Edexcel GCSE grade boundaries can be found here:
http://qualifications.pearson.com/en/support/support-topics/results-certification/grade-boundaries.html?Qualification-Family=GCSEBut note that this summer's exam boundaries aren't decided until all exams have been marked. As an example, if we look at the grade boundaries for the theory exam from June 2015, you needed 60/80 for an A* (which converted to 72/80 UMS - i.e. 90%), 56/80 for an A (which converted to 64/80 UMS - i.e. 80% etc.).
http://qualifications.pearson.com/content/dam/pdf/Support/Grade-boundaries/GCSE/1506-GCSE-Grade-Boundaries.pdfThe point is that you generally don't need as high amount of marks in the exam as you think to get the grade you want.
If you have got 50/50 in the practical, this converts to 120 UMS. The overall qualification is out of 200 UMS, so 120/200 = 60% = a C grade before you even walk into the exam.
To then get an A* having already got 120 UMS, you need another 60/80 UMS to make 180/200 UMS overall. To get 60/80 UMS last year (a grade B in the exam) you only needed 54/80 raw marks in the exam.