You will get many varied opinions on this as arguments can be made for several different setups.

I run 5Heavy-1Light-1Medium to get the most stat bonuses as a large stam pool means you can sustain block longer, as well as having a large magicka pool for support skills. Also DK tanks have several percentage based resource recovery skills like Mountains Blessing (5% stam when using earthen heart) or Battle Roar (% resources returned based on Ultimate cost). Since these are percentage based, the larger the resource pool, the larger the return.

If you have low CP or trouble with avoiding damage going 6-1 or 7 heavy may be warranted.

I would probably only run two medium if I was using the alkosh set, as you have no other options, unless you get 2x sword and shield (hard to come by at the moment). If you are a DK, magicka sustain is more beneficial so 6-1 HA/Light, or even two light.

The stam recovery bonuses from medium armor are voided when blocking. They will reduce the cost of your stam abilities, but as a tank you should be mindful of your stamina and not spamming abilities. Only apply Taunt every 10 seconds or so, only apply Heroic Slash after the duration has expired etc.