Ans to a and b) There's no limit to the dividend you can pay. However you need to strike a balance between the business' working capital needs and the amount being paid out as dividend to shareholders.
What determines the leval of dividends is the level of distributable reserves - that is to say accumulated company profits after allowing for tax. You will probably need your accountant to help you determine this to start with to show you how its done.
Your company tax year is completely separate from your personal tax year although it may be the same (this is actually best avoided as it cuts down on opportunities for tax panning).
What is the "best" amount for you to take will depend entirely on your circumstances. Again back to your accountant for that one.
Hope that helps
Regards,
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James Smith Chartered Accountant www.jamesesmith.co.uk
01235 536773