I would advise you to have the blood test on day 2 or 3 for FSH and oestradiol as this will give the most accurate result.  If this is difficult because of your cycle starting date you could ask if you could have your blood test taken elsewhere.  I am sure your GP will help you in this matter.  Alternatively you could have another test, AMH (Anti Mullerian Hormone) which is another measure of ovarian reserve and can be taken on any day of the cycle.