Technique for Concentrative Meditation
The term 'shamatha' means 'tranquility' or 'calm abiding' rather 'concentration' ('collectedness'; non-distraction). 'Shamatha' is a fruit of concentration rather than concentration itself.

It does not matter if you are also capable of simply paying attention to the object while thoughts emerge &amp; can redirect attention to the object when thoughts have done so. The very fact that thoughts are emerging is a loss of concentration, however momentary. 

Generally, the kinds of thoughts that help meditation are wise thoughts used to counter unwise thoughts and the very subtle thought (intention) of giving up (letting go) or non-attachment.

Some subtle thoughts can serve as objects of insight (vipassana) into impermanence but there is generally no need to deliberately cultivate such thoughts.